CS177 Python Programming - Purdue Universityfall14:rec11_f14.pdf · Print the first element Print...

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CS177 Python Programming

Recitation 11 – Data Collections

Table of Contents

• Review the use of lists (arrays) to represent a collection of related data.

• Review the functions and methods available for manipulating Python lists.

• Review the use of other data collections in Python, such as dictionaries and tuples.

CS17700 Programming With Multimedia Objects

Sequences

• When we have multiple elements stored consecutively in memory we call it a List

• When we have multiple characters stored consecutively in memory we call it a String

• Both of these structures are sequence structured (individual items can be selected by indexing i.e. s[i]).

• Ranges, Lists and Strings are 0 indexed

• Lists are defined by []

• Lists can contain strings, numbers, even other lists.

>>>x = “ABCD” >>>y = [“A”,”B”,”C”,”D”] >>>print (x) ABCD >>>print (y) ['A', 'B', 'C', 'D']

Print a string (A string of characters)

Print a list (A list of characters)

• Lists are more “general purpose”

– Allow heterogeneous elements in the same list

>>> myList = ["X", "B", 3, "A", 1]

>>> print (myList)

['X', 'B', 3, 'A', 1]

>>> myList = [['X', 'B', 3, 'A', 1], ‘hello’, 99]

[] Notation in Lists

• a[i] : gives a name to the ith element of a sequence

• The [] can be used to index into lists, ranges, or strings.

• If the sequence is a list, e.g., a = list(range(0, 10))

– a[i] is equal to the i+1 number in the range of 0 to 9

(index starts from 0)

Lists Operations

Operator Meaning

<seq> + <seq> Concatenation

<seq> * <int-expr> Repetition

<seq>[] Indexing

len(<seq>) Length

<seq>[:] Slicing

for <var> in <seq>: Iteration

<expr> in <seq> Membership (Boolean)

Slicing >>>x = list(range(0,10)) >>>print (x[0]) 0 >>>print (x[0:5]) [0,1,2,3,4] >>>print (x[:3]) [0,1,2] >>>print (x[3:]) [3,4,5,6,7,8,9] >>>print (x[-1:]) [9] >>>print (x[:-1]) [0,1,2,3,4,5,6,7,8]

Print the first element

Print the first five elements

Print the first three elements Starting point is omitted, default value is 0

Print from the fourth element until the end Ending point is omitted

Print from the last element until the end Ending point is omitted

Print from the first element to the last Starting point is omitted, default value is 0

Get element!

Get sublist!

Examples of List Operations

>>> print([1,2] + [3,4])

>>> print([1,2]*3)

>>> grades = ['A', 'B', 'C', 'D', 'F']

>>> print(grades[0])

>>> print(grades[2:4])

>>> print(len(grades))

>>> print([1,2] + [3,4])

[1, 2, 3, 4]

>>> print([1,2]*3)

[1, 2, 1, 2, 1, 2]

>>> grades = ['A', 'B', 'C', 'D', 'F']

>>> print(grades[0])

>>> print(grades[2:4])

['C', 'D']

>>> print(len(grades))

Lists Operations (con’t)

Method Meaning

<list>.append(x) Add element x to end of list.

<list>.sort() Sort (order) the list. A comparison function may be passed as a parameter.

<list>.reverse() Reverse the list.

<list>.index(x) Returns index of first occurrence of x.

<list>.insert(i, x) Insert x into list at index i.

<list>.count(x) Returns the number of occurrences of x in list.

<list>.remove(x) Deletes the first occurrence of x in list.

<list>.pop(i) Deletes the ith element of the list and returns its value.

>>> a=[]

>>> for i in range(15, 3, -2):

a.append(i)

>>> print(a)

>>> print(a.reverse())

>>> print(a.index(7))

>>> a=[]

>>> for i in range(15, 3, -2):

a.append(i)

>>> print(a)

[15, 13, 11, 9, 7, 5]

>>> print(a.reverse())

[5, 7, 9, 11, 13, 15]

>>> print(a.index(7))

>>> a.insert(2, 15)

>>> print(a)

>>> print(a.count(15))

>>> a.remove(15)

>>> print(a)

>>> print(a.pop(2))

>>> a.insert(2, 15)

>>> print(a)

[5, 7, 15, 9, 11, 13, 15]

>>> print(a.count(15))

>>> a.remove(15)

>>> print(a)

[5, 7, 9, 11, 13, 15]

>>> print(a.pop(2))

Dictionaries, Sets, Tuples

• A collection of unordered values accessed by key rather than by index is called Dictionary

• A collection of unordered and non-duplicated elements is called Sets

• In Python a Tuple is much like a list except that it is immutable (unchangeable) once created, i.e. (42, 56, 7)

• Note: Being an unordered collection, dic/sets do not record element position or order of insertion. Accordingly, indexing, slicing, or other sequence-like behavior are not supported.

Strings, Lists, Dictionaries, Sets, Tuples

Data Type Description

List Sequential, ordered, can have duplicates, mutable, i.e. [‘h’, ‘e’, ‘l’, ‘l’, ‘o’]

String Sequential, ordered, can have duplicates, immutable, i.e. “hello”

Dictionary Non-sequential, non-ordered, unique keys, can have duplicates, mutable, i.e. {1:‘h’, 2:‘e’, 3:‘l’, 4:‘l’, 5:‘o’}

Set Non-sequential, non-ordered, non-duplicate, mutable, i.e. setset(('h','e', 'l', 'o'))

Tuple Sequential, ordered, can have duplicates, immutable, i.e. tuple(('h','e', 'l', 'l', 'o'))

Dictionary Methods

• A dictionary is an unordered set of key: value pairs

• len(d)

Return the number of items in the dictionary d.

• d[key]

Return the item of d with key key.

Raises a KeyError if key is not in the map.

• d[key] = value

Set d[key] to value.

Dictionaries Methods

•del d[key]

Remove d[key] from d.

Raises a KeyError if key is not in the map.

•key in d

Return True if d has a key key, else False.

>>> d = {} #empty dictionary

>>> d = {‘date’ : 18}

#set ‘date’ maps to 18

>>> d[‘date’] = 20

#change the value mapped to by the key ‘date’ to 20

Example

Is this right??

If yes, what is the output?

If no, why?

>>> d = {'alice' : 1, 'bob' : 2, 'calie': 1}

>>> d = {'alice' : 1, 'bob' : 2, 'alice': 3}

Example

Given a dictionary dic and a list lst, remove all elements from the dictionary whose key is an element of lst. For example, given the dictionary {1:2, 3:4, 5:6, 7:8} and the list [1, 7], the resulting dictionary would be {3:4, 5:6}. Assume every element of the list is a key in the dictionary.

Example

Given a dictionary dic and a list lst, remove all elements from the dictionary whose key is an element of lst. For example, given the dictionary {1:2, 3:4, 5:6, 7:8} and the list [1, 7], the resulting dictionary would be {3:4, 5:6}. Assume every element of the list is a key in the dictionary.

for e in lst :

del dic[e]

Sets Methods

• S.update(t) Return set S with element added from t

• S.add(x)

Add element x to set S

• S.remove(x)

Remove x from set S, raises KeyError if not present

>>> engineers = set(['John', 'Jane', 'Jack', 'Janice'])

>>> engineers.add('Marvin')

>>> employees = set()

>>> employees.update(engineers)

>>> employees.remove('Jack')

Example

>>> engineers = set(['John', 'Jane', 'Jack', 'Janice'])

>>> engineers.add('Marvin')

{'Jack', 'Marvin', 'Janice', 'John', 'Jane'}

>>> employees = set()

>>> employees.update(engineers)

{'John', 'Jane', 'Janice', 'Jack', 'Marvin'}

>>> employees.remove('Jack')

{'John', 'Jane', 'Janice', 'Marvin‘}

Example

Examples

Given the string line , create a set of all the vowels in line. Associate the set with the variable vowels.

Examples

Given the string line , create a set of all the vowels in line. Associate the set with the variable vowels.

vowels = set()

for x in line:

if x=="a" or x=="e" or x=="i" or x=="o" or x=="u":

vowels.add(x)

Tuples

>>> t = (12345, 54321, 'hello!' )

>>> print (t)

(12345, 54321, 'hello!')

>>> # Tuples may be nested: ...

>>> u = (t, (1, 2, 3, 4, 5))

>>> print(u)

((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))

Examples

>>> # Tuples are immutable: ...

>>> t[0] = 88888

Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'tuple' object does not support item assignment

>>> # but they can contain mutable objects: ... v = ([1, 2, 3], [3, 2, 1])

>>> v[1][2] = 99

>>>print(v)

([1, 2, 3], [3, 2, 99])

Examples

Given that t has been defined and refers to a tuple write some statements that associate with t a new tuple containing the same elements as the original but in sorted order.

Examples

Given that t has been defined and refers to a tuple write some statements that associate with t a new tuple containing the same elements as the original but in sorted order. tmp = list(t) tmp.sort() t = tuple(tmp)

CS177 Python Programming - Purdue Universityfall14:rec11_f14.pdf · Print the first element Print...

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